Underground infrastructure and asset location includes and is the process of locating, identifying, and labeling assets which are buried below the earth's surface at varying depths. These assets may include survey markers, communication lines, power distribution, natural gas, cable television, fiber optics, storm drains, water mains, and wastewater pipes, etc. These systems are often run underground; some by the very nature of their function, others for convenience or aesthetics.
Before digging, local governments often require that the underground system's locations be denoted and approved (e.g., if it is to be in a public right-of-way). Also, owners commonly require location and identification of assets to facilitate management and maintenance. Because of the many different types of materials that go into manufacturing each of the different types of underground assets, different detection and location methods are typically used. For metal pipes and cables, magnetic locators or electromagnetic equipment consisting of a transmitter and a receiver are often utilized. For other types of pipe, such as those made of plastic or concrete, other types of radio or ultrasonic location systems are commonly required. Location by these technical means is often necessary because maps of subsurface assets tend to lack the pinpoint accuracy and/or precision needed to ensure proper clearance or facilitate maintenance or repair. This is especially an issue in older or remote areas, any maps of which may be dated and/or inaccurate, or may be missing entirely. In limited cases, a few utilities and assets are “permanently” marked with short exposed posts which are vulnerable to damage or accidental removal.
Radio frequency identification (RFID) tags are well-known and typically include an integrated circuit that is operatively coupled to an antenna. The integrated circuit typically includes some amount of memory in which a tag identifier is stored, with possibly other information related to the tag and/or the item(s) with which the tag is associated. When an RFID reader or interrogator transmits energy via its reader antenna to interrogate the RFID tag, the tag responds with information from which the reader can obtain the RFID tag identifier and any other information. The identifier and other information may be utilized to determine characteristics of the RFID-tagged items(s). The tag may also have a battery, or it may have no battery and be powered by a capacitor using energy from an external reader.
RFID tags typically operate in low frequency (less than 100 MHz) or high frequency (more than 100 MHz) modes. High frequency tags can have their data read at greater distances that lower frequency tags. Ultra-high frequency (UHF) tags are a subset of high frequency tags and operate in a range of higher frequencies between 300 MHz and 3 GHz (3000 MHz), also known as the decimeter band or decimeter wave as the wavelengths of UHF waves range from one to ten decimeters (10 cm to 1 meter).
Ultra-high frequency (UHF) radio frequency identification (RFID) is a proven technology that has been used been used extensively above ground as a logical replacement to the aging barcode system. UHF RFID has taken over identification of much of the retail product market and is used for tracking location and status of product in all phases of manufacture and sales. This explosive adoption has been possible due to technological advances resulting in low hardware cost relative to the exceptional improvement in inventory control and tracking.
In a typical UHF RFID system, a reader and tag communicate using variations in amplitude of reflected signals called “backscatter modulation” or backscatter communication. With backscatter communication, the radio frequency wave must propagate both from the reader to the tag (Forward Link) where it is used to energize the tag, and then a portion is reflected back from the tag to the reader (Reverse Link). Backscatter communication results in a reduction in the power density (strength) of an electromagnetic wave as the wave propagates through space. This is called path loss. Path loss may be due to many effects, such as free-space loss, refraction, diffraction, reflection, and absorption. Path loss is largely influenced by environment, propagation medium, the distance between the transmitter and the receiver, and the relative location of the antennas.
These losses affect both the signal transmitted from the reader to the tag (Forward Link) and the backscattered return signal from the tag to the reader (Reverse Link) and limit read range such that tags buried in depths may be subject to interference that renders them unreliable.
As such, UHF RFID tags are not typically used below ground. Radiated UHF RF waves do not penetrate soil, earth, or water well, and the higher frequencies are attenuated and inhibited more such that transmission of such waves through the earth has been generally impracticable. In the case of reading tags buried in the soil, the soil properties and moisture content play a significant role in signal attenuation and maximum read depth.
Because they are not typically used below ground, conventional UHF RFID tags are adapted or configured to communicate in air. Such tags typically use and include a patch antenna because it is simple to fabricate, easy to implement, low profile and compact, and light weight. Such tags work well for typical above-ground tracking and location uses such as logistics and supply chain management, item level inventory tracking, race timing, attendee tracking, materials management, access control, IT assets tracking, tool tracking, library materials tracking, etc.
Such known UHF RFID tags with patch antennas may be sensitive to interference and, depending upon several soil parameters including permittivity, permeability, and resistivity, may have limited read range potential when buried below the Earth's surface or provided in or around other various lossy mediums (e.g., underwater, concrete walls, wood beams, etc.). This is especially true of known passive UHF RFID tags. The UHF RF signal is attenuated by both soil surface reflection loss and adsorption loss due to the soil properties and electrolyte content.